1. Field of the Invention
This invention relates to electrochemical cells, and more particularly to stacks of fuel cells having external reactant gas manifolds.
2. Description of the Prior Art
One type of fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in the space between the two electrodes; each electrode also includes a catalyst layer on the electrolyte side thereof. On the nonelectrolyte side of the anode electrode is a reactant gas passage for carrying a fuel, and on the nonelectrolyte side of the cathode electrode is a reactant gas passage or chamber for carrying an oxidant. The electrodes are constructed so that the reactant gas diffuses therethrough and comes into contact with the electrolyte in the catalyst layer thereby causing an electrochemical reaction whereby ions travel from one electrode to the other through the electrolyte and electrons travel from one electrode to the other via an external circuit. The flow of electrons is the useful electric current produced by the cell. In an electrolysis cell the opposite occurs. Electrical current is supplied to the cell and gases are generated at the electrodes.
In a fuel cell powerplant a plurality of fuel cells are connected electrically in series through electrically conductive, gas impervious plates separating adjacent cells, thereby forming a stack of fuel cells. These separator plates, in combination with the electrodes adjacent thereto, generally define the reactant gas passages hereinbefore referred to.
There are two basic approaches for feeding reactant gases to the individual cells within the stack. These approaches are via internal manifolding and external manifolding. Representative of internal manifolding is U.S. Pat. No. 3,012,086. Representative of external manifolding is commonly owned U.S. Pat. No. 3,994,784. In the case of internal manifolding, the reactants are fed to the stack via channels passing through the cells in a direction perpendicular to the plane of the cells. These internal perpendicular channels communicate with the cell reactant gas passages which are generally oriented parallel to the plane of the cell. The peripheral portions of each cell component abut the peripheral portions of adjacent components and form continuous seals around the outer edge of each cell and between adjacent cells so that reactant gases and electrolyte cannot escape from within the cells. An axial (i.e., perpendicular to the plane of the cells) loading system is used to ensure adequate sealing and good electrical and thermal conductivity between adjacent cells and components. Typically the cells are compressed between a pair of end plates using tie bolts to interconnect the end plates and urge them toward one another. Any desired compressive force can be applied to the stack by suitable tightening of the tie bolts.
One problem associated with maintaining a compressive load on a stack of fuel cells is the difference between the thermal expansion characteristics of the stack and the axial loading system, in combination with the creep characteristics of conventional cell components. Generally the axial loading system has a higher coefficient of thermal expansion than the stack materials, resulting in a loss of compressive load upon heating the stack. Creep of the cell components with time also results in reduced compressive forces on the stack. A stack of cells which is not particularly resilient may not be able to tolerate excessive thermal expansion mismatch since the compressive loads may fall off to the point where sealing and electrical contact between components and between cells is not adequate. Creep of the cell stack presents a similar but long term problem. Of course, the tie bolts could be tightened down periodically although the necessity for doing this is not desirable.
A solution to this problem is a mechanical load follow-up system built into the axial loading system for the purpose of trying to maintain the compressive load relatively constant as the stack height changes due to thermal expansion mismatch and creep. Commonly owned U.S. Pat. No. 3,253,958 describes one such system. While no known load follow-up system is perfect, they do significantly reduce the extent to which the compressive load would otherwise drop off during operation and with time.
Although axial load follow-up systems work well in some situations, they are complex, expensive, and not desirable for large stacks with external reactant gas manifolds. Referring again to U.S. Pat. No. 3,994,748, external manifolds are used with fuel cells having reactant gas passages extending from one edge of the cell to the opposite edge of the cell. The reactant gas is fed into these passages from a manifold on one side of the stack, travels through the cells and exits from the passages into a manifold on the opposite side of the stack. The inlet manifold covers an entire side of the stack and must be sealed around its outer edge against the side of the stack. The same is true for the outlet manifold. In a stack of several hundred cells, thermal expansion and, in particular, creep can result in a change of stack height on the order of two inches or more, depending on the size of the stack. It is very difficult to maintain an effective seal around the edge of the manifold where there is that much relative movement between the cells and the manifold, such as is the case with an axial load follow-up system which permits the stack height to change with time due to creep. Of course, stacks with internal reactant gas manifolding do not have this problem, however, they are inherently more expensive than external manifolding.